In electron beam lithography, a polymeric film which is sensitive to light, generally termed a resist, is applied to a support substrate. The film is then exposed to radiation which defines a pattern desired to be reproduced on the support. Exposure of the resist results in a different solubility between the exposed and unexposed portions so that a solvent can be used to remove the more soluble portion of the resist and a surface relief pattern can be obtained. Thereafter by using suitable etching techniques desired features can be produced on or in the substrate. As described for example in various publications and U.S. Pat. No. 4,262,083, an electron beam can be used to delineate circuits with greater resolution than that which can be obtained with light in prior art photo lithographic processes. The higher resolution of the electron beam is due in part to the fact that an electron beam can be more precisely focused than can a light beam. The electron beam has been proven commercially and continues to be a viable and important tool for the manufacture of microelectronic circuits and masks for forming printed circuits by other methods.
An important factor which has inhibited the full utilization of electron beam and X-ray lithography to date in the manufacture of electronic circuits are limitations on the resists which are used in the process.
Various polymeric compositions have been suggested as candidates for resists. Commercially available electron beam resist polymeric compositions include polystyrene, polymethyl methacrylate, polyglycidyl methacrylate and polychloromethylstyrene. Other polymers which may be used include poly(diallylphthalate), poly(methylvinyl ketone), poly(vinylacetate), poly(vinyl cinnamate), poly(vinyl chloride), polyvinyl pyrrolidone, poly(glycidyl methacrylate-co-ethyl acrylate), poly(4-chlorostyrene), polyvinyltoluene (mixed isomers), polyvinylbenzylchlorides, poly (4-bromostyrene), poly(4-iso-propylstyrene), poly(4-tert.-butylstyrene), poly(butene-1-sulfone), copolymers of alpha cyano ethyl acrylate and alpha amide ethyl acrylate, poly(glydidyl acrylate), poly (glycidyl methacrylate-co-3-Chlorostyrene), as well as similar polymers.
The advantage in using electron resists composed of high molecular weight polymers is that the polymers generally undergo cross linking or chain scission as a result of the action of the high energy electrons on the polymer chain itself or on the radiation sensitive chemical moieties attached to the polymer chain. The glycidyl methacrylate family of resists are typical of this latter type of material. In compounds of this type the radiation-labile epoxy moiety is readily converted into a cross linkable radical type and species by the action of the electrons. In general, any polymeric material useful as a negative resist cross-links and becomes insoluble on exposure to electrons.
When an electron beam enters a thin polymer film, the incident electrons lose energy as they undergo inelastic collisions with the polymer molecules. Electrons used in electron beam lithography, typically on the order of 5-20 keV, have energy several orders of magnitude in excess of that associated with chemical bonds. The electron-chemical bond interaction is relatively non-specific and not strictly analogous to the well characterized reactions of molecular photochemistry. For this reason most of the investigators have been hesitant to discuss resist action in terms of specific chemical reaction mechanisms. Notwithstanding the chemical nature of electron interaction with polymers, the initially formed reactive species do follow the generally well known rules of chemical reactivity. This means that the nature of the reactive intermediates can lead to suggested improvements in resist properties by chemical tailoring.
Resist materials for use in electron beam lithography must possess a number of characteristics. Thus, they must be soluble in a solvent so that they can be applied as a thin film on the supporting substrate and must have sufficient adhesion to form a tight bond with the substrate. Further, as a result of exposure to the selected type of radiation, a substantial change in the solubility of the exposed and unexposed portions must occur so that one of the portions will become soluble in a selective solvent and can be removed to provide the desired surface relief pattern. Depending on whether or not the resist is a negative resist or a positive resist will determine whether or not the resulting soluble portion is the exposed or unexposed area.
The resist materials, in addition to having the above characteristics, must also have the required degree of sensitivity, resolution and contrast to define submicron patterns required by the process. These particular characteristics are in addition to those essential requirements that they provide resists free from defects, be uniform, be easy to process, be resistant to degradation by the etchant, and subsequently easy to remove. In general, however, most of the previously known commercial resist materials fall short in one or more of the criteria deemed necessary for compatibility with electron-beam lithography.
Most of these previously described resist materials suffer from the lack of sensitivity, the lack of resolution, or the lack of resistance to plasma etching. For example, poly(butene-1-sulfone), or PBS, resist is sensitive, has high resolution, but does not withstand a plasma etching. However, a poly(glycidyl methacrylate-co-ethyl acrylate), resist is sensitive, withstands plasma etching moderately, but has relatively lower resolution. Poly(methyl methacrylate) and polystyrene have high resolution and great ability to withstand plasma etching but are very insensitive. One simple expedient which would improve the plasma etch durability of a resist image is to use a thicker initial resist coating on the substrate. Unfortunately, however, ultimate resolution is then worse when a thicker coating is used and there are increased problems with resist image swelling.
An alternative approach to this problem is to chemically modify the polymer materials used as resists according to known relationships between structure and properties to improve sensitivity or improve plasma etch durability. Thus, it is known that the sensitivity of polystyrene is improved by the incorporation of chloromethyl moieties as shown by Imamura, J. Electrochem. Soc., 126, 1628 (1979), and Choong et al, 16th Symposium on Electron Ion and Photon Beam Technology, Dallas, Texas, May 26-29, 1981. Also, the resist (2,2,2-trifluoroethyl-alpha-chloroacrylate), has greater sensitivity than PMMA and improved etch resistance under exposure to moist air plasma as shown in product literature under the tradename EBR-9 by Toray Inc., Tokyo, 1981.
Moreover it is known that a resist material may be compared to any polymeric organic coating and its physical and chemical properties can be modified by following well-established procedures used extensively by the coatings industry. For example, thermal and radiation stability can be improved by incorporating aromatic moieties into the polymer. Polystyrene has been used by workers of Texas Instruments as a negative e-beam resist in the fabrication of microwave bipolartransistors. Sensitivity is worse than 10.sup.-5 c/cm.sup.2 but polystyrene exhibits a high contrast and serves as a tenacious etching mask. Plasma etch durability is reported to be excellent, see Thompson, Solid State Technology, page 27, July 1974. Polyglycidyl methacrylate (PGMA) is also known to be a sensitive resist with only fair plasma etch durability. It has been found that glycidyl methacrylate monomer can copolymerize with styrene to create a resist with improved sensitivity over polystyrene, while incorporating much of polystyrene's plasma etch durability, see Thompson et al., J. Electrochem. Soc., 126, page 1699 (1979). Also it is known that glycidyl methacrylate may be copolymerized with chlorostyrene to result in a modified resist.
While incorporation of aromatic chemical moieties in a polymeric material has been demonstrated to improve plasma etch durability the more durable resists are not as sensitive as the current commercially available e-beam resists. The present invention solves the problem in the art of providing a process wherein the plasma etch durability of resist images delineated in the current generation of commercially available electron beam sensitive resist compounds can be substantially improved.